As a depth detection technique that can be applied to a digital camera, a depth detection technique to detect a depth to an object by a phase difference method, with providing a depth measurement function to a part or all of the pixels of an imaging device, is known. A pixel having the depth measurement function (hereafter called “depth measurement pixel”) includes a plurality of photoelectric conversion units which receives luminous flux that passed different regions on a pupil of an imaging optical system respectively. The depth is measured by estimating a shift amount of the image signal generated by each conversion unit, and converting the shift amount into a defocus amount using a conversion coefficient.
A cause of a drop in the depth measurement accuracy is the change of profile of an image signal generated by each photoelectric conversion unit. If the image signal generated by each photoelectric conversion unit has a mutually different profile, due to the vignetting of luminous flux caused by a frame of the optical system or the like, and the difference of sensitivity characteristics among the pixels, the accuracy of estimating the shift amount of the image signal drops, and depth measurement accuracy drops as well. Patent Literature 1 discloses a method for correcting the profile of the image by performing image signal correction filtering on the image signal, whereby the detection accuracy of the shift amount is improved.
Another cause of a drop in the depth measurement accuracy is fluctuation of the conversion coefficient. The conversion coefficient fluctuates depending on a defocus amount and a position on the imaging device for which depth is measured. If the shift amount is converted into the defocus amount using a fixed conversion coefficient regardless the depth measurement conditions, a conversion error occurs. Patent Literature 2 discloses a method for reducing the depth calculation error by calculating the conversion coefficient according to the value of the defocus amount.